Volume control circuit



y 1935. H. A. AFFEL VOLUME CONTROL CIRCUIT Filed May 5, 1934 J L aiwwl IHHH INVENTORY H. AAFFEL A rromvsr rier wave transmission system that shall vary the Patented July 9, 1935 VOLUME CONTROIi' omourr Herman Andrew Affel, Ri dgcwood, N. J., assignor to American Telephone andTelegra'ph panma corporation of New York p Com- pplication May 5, 1934; Serial N mnmm Claims. 11. 250-43),

This invention relates to carrier wave transmission systems and particularly to volume control of carrier wave transmission systems.

.- One object of the invention isto provide a carvolume of a carrier wave transmitted to a receiving station and that shall employ the variable-volume carrier wave, to control the volume of the demodulated signals at a receiving station.

Another object of the invention is to provide a transmission system, that shall transmit to. a receiving station a constant volume side band and a carrier wave varying .in volume according to 4 vide automatic volume control to'take care of the so-called fading. The automatic volume control at the receiving-station is generally governed by the carrier wave. Moreover, a volume range .compr'ession-expansion has in effect sometimesrbeen provided'by use of a variable carrier so that the receiver will be less subject to noise during the intervals when the programs are of a low volume. r The employing of a'variable carrier for preventing disturbance in a receiver when the signal volume is low makes it impossible to obtain automatic volume control by means of the carrier wave. 1 e In accordance with the present invention it is proposed to provide for volume control and volume compression and expansionpwithout em-' ploying anyadditional control channel. The carrier wave is controlled to obtain the volume range compression-expansion feature and one of the side bands is held at substantially constantvolume from the transmitting stationxto obtain automatic volume control. p .1 The transmitting circuitin the present invention is essentially a constant side band variable carrier transmitter. Because of.-thejdifiiculty in separating the'carrier wave and the side bandsat thejhigh frequencies, of radiotransmission, a

double modulation system is disclosed. It is to be understood that the invention is not limited to such a type of system. I

In .a' transmitting station constructed in accordance with the invention a suitable modulator is provided for modulating a carrier w'ave with the received signal waves which are to be trans mittedto a distant receiving station. One side band from the modulator output is selected and supplied to a second modulator. A second carrier'wave is modulated with the selected side band in a second modulator and the output supplied to a'control amplifier. The control amplifier is so governed as to maintain the output thereof at substantially constant volume. One side band from the second modulation operation is selected from the output of the control amplifier. for transmission to the distant receiving station. The signal waves'which are supplied to the first modulator are also supplied to a control amplifier for varying'the volume of the second carrier wave according to the volume variations of the signal waves. The second carrier wave with its volume varied according to the signal waves is transmitted with the selected side band toa distant receiving station either over wires or by radio. In the accompanying'drawing the invention is illustrated by means of a radio system.

At the receiving station the combined currents are first stepped'down by the heterodyne method to a suitable lower frequency for which the selecting circuits and amplifiers are more effective. The. combined side band and carrier wave are supplied to a gain control amplifier which is-controlled by the selected side band. In the output circuit of the gain control amplifier is a band selector for selecting the side band and a filter for selecting the carrier wave of variablevolume. A rectifier which is connected to the out; put side of the band filter" is connected to the gain control amplifier for controlling such am- .plifier to maintain the side band at substantially constant volume. 'The gain control amplifier corrects for the fading encountered during transmission. The output from the band filter and the output from the carrier filter after amplification are supplied to a demodulator. The demodulator is of a type which follows the square law so that the amplitude of the signals in the output 10f the demodulatorfollows the product of the carrier wave and the side band. Inasmuch as the side band is maintainedat substantially constant volume, the signals in the de modulator output circuit must follow the volume 2 and 3 has the input circuit thereofv connected,

by a transformer 4 to an oscillator 5. The input circuit of the modulator is also connected. by

a transformer 6 to a circuit 1 which supplies signals from a microphone or from a telephone cir-" cuit. A band filter 8 is connected to the output circuit by a transformer 9. The band filter 8 which selects one side band from the modulator output circuit is connected'to a second modulator l0 by means of a transformer The second modulator I0 is preferably of the push-pull type and comprises space discharge devices l2 and I3. The modulators I and Illfare preferably of the push-pull type in order to eliminate the carrier wave. An oscillator M which supplies the second carrier wave .is connected to the input circuit of themodulator In by means of a transformer IS. The oscillator I4 is also connected to the input circuit of a 'controlamplifier It by means of a transformer l1. The output circuit of the modulator Hi containing the'side band is connected by a'transformer IE to a control amplifier l9. The amplifier I9 is so controlled as to maintain theside bands in the output circuit thereof at substantially constant volume. 'A transformer connects the output circuit of the control amplifier I!) to a band filter 2| which selects one of the side bands for transmission to a distant receiving station. The transformer 20 also connects the output circuit of the control amplifier 19 to a rectifier 22. Current from the rectifier22 is impressed'on a potentiometer 23 which supplies grid bias through a resistance 24 for controlling the amplifier IS. A condenser 25 is connected across the potentiometer23 for smoothing out the rectified current. The output from the rectifier 22 so controls the bias on the gridof the'control amplifier l9 as to maintain the side band output from the amplifier at substantially constant volume.

The amplifier i6 which is connected to the oscillator |4' by means of a transformer I1 is controlled in accordance with "the volume variations of the signals supplied to the first modulator A rectifier 21 which is connected to the signal circuit 1 supplies rectifier current to a potentiometer 28. The potentiometer 23 supplies grid biasing potential through a resistance element 29 to the control amplifier IS. A condenser 30 is connected across the potentiometer 23 for smoothing out'the rectified current. The control amplifier l6. which effects variations in the volume of the carrier wave from the oscillator |4 according to thevariations in volume of the'signals supplied to the modulator I is connected to-a filter 3| by means of a transformer 32'. The carrier wave of variable volume in the output circuit of the filter 3| is supplied to the output circuit of the band filter 2|. The selected side band andthe carrier wave of variable volume are amplified by theamplifier 33 and transmitted" by radio to a distant receivingstation.

Referring to Fig. 2 of the drawing a heterodyne modulating stage is shown comprising a space discharge device 35 having the input circuit there of connected by a transformer 56 to an oscillator 51 and by suitable coupling to an antenna. The Waves received from the transmitting station shown in Fig. 1 are reduced to a suitable lower frequency by theheterodyne modulator. The output circuit of the heterodyne modulator 35 is connected by a transformer 36 to a gain control amplifier 31. The gain control amplifier 3'! is controlled by the received side band to correct for fading effected during transmission. The output circuit of the gain control amplifier 31 is connected by a transformer 38 to a band filter 33 which selects the transmitted side band and to a filter 40 which selects the variable-volume carrier wave. A rectifier 4| which is connected to the output circuit of the band filter 39 impresses potential across a potentiometer 42. The potentiometer 42 supplies'grid bias through a resistance element 43 to the gain control amplifier 31. By means of the rectifier 4| and the circuit controlled thereby the gain control amplifier 31 is so governed as to maintain the side band in the output circuit thereof at substantially constant volume.

The output-circuit of the band filter 39 is connected to the input circuit'ofa demodulator 44 by means of a transformer 45; The demodulator 44 is preferably of the push-pull type and comprises space discharge devices 46 and 41. The filter 40 which selects the variable-volume carrier wave is connected to an amplifier 48 which, in turn, is connected by a transformer 49 to the input circuit of the demodulator 44. The space discharge devices 46 and 4! in the demodulator 44 follow the "square law so that the signals in the output circuit'are proportional to the product of the side band and the carrier wave. The side band has been maintained at substantially con stant volumeso that the output signals have volume variations according to the variations in volume of the carrier wave. The output from the demodulator 44 is connected'by a transformer 50 team amplifier 5|. The amplifier 5| is connected to a suitable loud speaker 52.

In the above described system it will be noted that the advantages of a volume control system are obtained by the control effected through the selected side band and that the advantages of a compression expansion system are effected by controlling the volume of the transmitted carrier. Moreover, no additional channel is required to effect the volume control as well as the volume compression and expansion features.

Modifications in the system and in the arrangements and location of parts may be made within the spirit and scope of the invention and such modifications are intended to be covered by the appended claims.

What is claimed is:

1. The method of transmitting signals which consists in modulating a constant volume carrier wave with signal waves, in deriving a constant volume side band from said modulation operation', in controlling the carrier wave according to the volume of said signals, in transmitting the constant volume side band and the variable-volume carrier wave to a receiving station, in correcting the received side band for fading effected during transmission and in demodulating the received side band to obtain signals varying in volume according to the received carrier wave;

I 2. The method of transmitting signals which consists in modulating a constant-volumecarrier and carrier wave for fading during transmission,

and in demodulating the received side band to obtain signals varying in volume according to the received carrier wave.

'3. The method of transmitting signals which consistsin deriving a constant-volume side band from a constant-volume carrier wave and signal waves, in varying the volume of the carrier wave according to the volume of the signal waves in transmitting the constant volume side band and the variable-volume carrier wave to a receiving station, in correcting the volume of the side band and the carrier wave for fading during transmis sion, and in demodulating the received side band to obtain signals varying in volume according to the variable volume carrier wave.

4. In a signal system, means for modulating a constant volume carrier wave with signals of variable volume, means for controlling the output means at the receiving station controlled by the variable-volume carrier wave for demodulating ,the constant volume side band to obtain signals varying in volume according to the variable-volume of the carrier wave i l 5. In a signal system, means comprising space discharge devices in push-pull relation for modulating a constant-volume carrier wave with variable-volume signal waves, gain control means for obtaining a constant-volume output from the modulation operation, means comprising a band filter for selecting one side band from the modulation output, means for varying the volume of said carrier wave according to the volume ofthe signals and for transmitting the selected side band and the variable-volume carrier wave to a receiving station, means for selecting the received side band, gain control means governed by i the received selected side band for correcting the i received side band and carrier wave for fading during transmission, and means at the receiving station controlled by the variable-volume carrier wave for demodulating the received side band to obtain signals varying in volumeaccording to the variable volume of the carrier wave.

6. In a signal system, means for generating a constant-volume carrier wave and for modulating the carrier wave with variable-volume signal waves, means for controlling the output of the modulation operation to render the volume thereof constant, means for selecting a side-band from the modulation output, means for varying the volfor fading during transmission, and means at the receiving station controlled by the variable volume carrier wave for demodulating the side band to obtain signals varying in volume according to variable volume of the carrier wave.

'7. In a signal system, means for generating a constant volume carrier wave and for modulating the carrier wave with signal waves, means for controlling the output of the modulation operation to render the volume thereof constant and for selecting one side band, means for controlling the volume of the carrier wave according to the volume of the signal waves and for transmitting the constant-volume side band and the variablevolume carrier wave to a receiving station, gain control means at the receiving station for correcting the received side band and carrier wave for. fading during transmission, and means for demodulating the side band under control of the variable carrier to obtain signalsvarying in volume according to the volume of the carrier wave.

8. In a signal system, the combination with a transmitting station comprising means for modulating'a carrier wave with signal waves, means for maintaining the volume of modulation output constant,'means for selecting a constant volume side band, means for controlling the volume of the carrier wave according to the volume of the signals and means for transmitting the selected side band and the variable-volume carrier wave,

of a receiving station comprising means for maintaining the volume of the received side band constant, and for demodulating the received side band to obtain demodulated signals which vary in volume according to the variable volume carrier] wave.

9. In a signal wave, means for modulating a constant volume carrier-wave with variable-volume signal waves, means for varying the volume of said constant volume carrier wave according to the volume variations of the signals, means for deriving a constant-volume side band from said modulation operation and for transmitting the constant volume side band and the variablevolume carrier wave to a receiving station, means at the receiving station for maintaining the volume of the received side band constant, and means controlled by the variable volume carrier wave for demodulating the side band to obtain signals varying in volume according to the variable volume carrier wave.

10. In a signal system, means at a transmitting 1 station comprising space discharge devices in push-pull relation for modulating a constant volume carrier wave with variablevolume signal waves, means for obtaining a constant volume output from the modulation operation, a band filter for selecting a side band from the constant volume ouput, means for varying the volume of the carrier wave according to the volume variations of the variable volume signals and for transmitting the constant volume side band and the variable volume carrier wave to a receiving station, gain control means at the receiving station for correcting the received side band and the'carrier wave for fading during transmission, and means for demodulating the side band under the control of the variable volume carrier to obtain signals varying in volume according to the volume of the carrier wave. 

